Wind turbine towers, especially tubular steel or concrete towers for large wind turbines, are large in diameter and weight. This may cause difficulties concerning the transportation of a tower to the wind farm and the used infrastructure. Usually, the steel or concrete towers for wind turbines are produced as sections in shops and the sections are then transported to the place of installation. The towers are typically constructed of a number of sections which have a cylindrical or conical shape. In the wind industry, the requirements for larger turbines have resulted in corresponding requirements for larger wind turbine towers. Larger wind turbine towers have typically lead to larger tower section diameters and longer and heavier tower sections. The dimensions of tall towers for large wind turbine have reached limits defined by the infrastructure of various countries. The limiting aspects are typically physical limits such as free height under bridges and tunnels, allowable weights or the turning radii of roundabouts.
The increasing number of turbines in large wind projects has also caused difficulties since the equipment which is needed to transport the largest tower sections by road or by rail is highly specialised and is not found in the quantities necessary for the present number of transportations. Consequently, when a large number of projects require a substantial amount of transportation time by road, the availability of special equipment may become difficult in the project phase.
The problem has been addressed by dimensioning, by the use of hybrid towers or by the use of modular towers. Dimensioning accepts the height and width restrictions of transportation routes and uses the restrictions as a design basis. This means in practice that the external tower diameter is fixed at a certain maximum value, typically 4.2 meters. When the diameter is fixed, then the wall thickness is dimensioned to provide the necessary stiffness and strength. For large turbines and tall towers this will typically lead to significantly higher weight. This causes higher costs compared with when no diameter restrictions are applied.
In a hybrid solution, the problem is circumvented by extending the concrete foundations significantly above ground level, for example, as a cylindrical structure of, for instance, 10 meters height. This increases the effective hub height of a wind turbine where the tower design is not significantly influenced by a diameter restriction. However, above a certain practical height an extended foundation is expensive. Compared with a diameter restricted tower, a hybrid solution tower reaches an additional height of perhaps 15 meters.
A wide range of modular precasted concrete towers are well known in literature and in practice. Using a longitudinal split, such solutions overcome the dimensional restrictions on transportation. However, difficulties occur in the assembly and the complexity of the modular elements.
WO 03/069099 A1 discloses a wind turbine comprising a stationary vertical tower on which the moving part of the wind turbine is arranged, which mast is at least partly composed from prefabricated wall parts with several adjacent wall parts forming a substantially annular mast part. WO 01/07731 discloses a tower manufactured by a slip form technique. This slip form technique uses vertical moving slip forms which are casting the whole circumference of tower in one process. However, it is difficult to vary the diameter of the tower.